Add waiters for elasticache

[Jc2k]

This adds 4 new waiters i've needed recently. I've used the same delays/attempts as RDS, which works for me quite well so far.

[coveralls]

Coverage remained the same at 94.1% when pulling 8c5987c on Jc2k:cache-waiters into 4a2163f on boto:develop.

[jamesls]

Thanks for adding these. The one thing I'd suggest changing is instead of using the 404 status code checker we use the error code returned by the service. There was a bug in botocore that prevented us from using this before, but this issue is now fixed. For example here's this change for RDS (https://github.com/boto/botocore/blob/develop/botocore/data/aws/rds/2014-10-31.waiters.json#L64-L68).

[Jc2k]

I saw you released the fix for that, and so quickly too! Cheers! I've made the change you requested.

[coveralls]

Coverage increased (+0.02%) to 94.12% when pulling de25c9b on Jc2k:cache-waiters into 4a2163f on boto:develop.

[jamesls]

Merged, thanks!